Pump assembly for obtaining a high vacuum

ABSTRACT

A pumping assembly for obtaining a high vacuum, the assembly comprising a primary pump (4) and a secondary pump (1) associated in series, the inlet of the secondary pump (1) being taken from an enclosure (3) to be evacuated, the assembly further including means (7) for starting the secondary pump (1) when the pressure upstream from the primary pump (4) drops below a value P 1 , the assembly being characterized in that a passive tank (10) followed by an isolating valve (11) are interposed between the outlet (12) from the secondary pump (1) and the inlet (13) to the primary pump (4), and in that it includes control means (7) for closing the isolating valve (11) and stopping the primary pump (4) when the pressure in said passive tank (10) reaches a value P 2  &lt;P 1 , and for opening the isolating valve (11) and restarting the primary pump (4) when the pressure in said passive tank ( 10) returns to the pressure P 1 .

The present invention relates to a pump assembly for obtaining a highvacuum.

It is well known that in order to obtain pressures of less than 10⁻³mbar, a primary pump is associated in series with a secondary pump. Whenthe assembly is started up, only the primary pump is run until thepressure upstream from the primary pump has dropped to a value P₁enabling the secondary pump to operate. The secondary pump is thenstarted and both pumps, i.e. the primary pump and the secondary pumpoperate simultaneously, in series, and permanently. The desired pressurein the enclosure is thus achieved after some length of time has elapsed.

Such a pumping assembly requires electricity to feed the motors drivingthe pumps. The electricity may be taken either from a mains supply orelse from a storage battery integrated in the pumping assembly.

The object of the invention is to economize the electrical energyconsumed during pumping operations. The invention is particularlyadvantageous for portable assemblies which are powered, in particular,from storage batteries, the invention making is possible to increase therunning time of the pumping assembly for a battery of given size andweight.

The present invention thus provides a pumping assembly for obtaining ahigh vacuum, the assembly comprising a primary pump and a secondary pumpassociated in series, the inlet of the secondary pump being taken froman enclosure to be evacuated, the assembly further including means forstarting the secondary pump when the pressure upstream from the primarypump drops below a value P₁, the assembly being characterized in that apassive tank followed by an isolating valve are interposed between theoutlet from the secondary pump and the inlet to the primary pump, and inthat it includes control means for closing the isolating valve andstopping the primary pump when the pressure in said passive tank reachesa value P₂ <P₁, and for opening the isolating valve and restarting theprimary pump when the pressure in said passive tank returns to thepressure P₁.

An embodiment of the invention is now described by way of example withreference to the accompanying drawing, in which:

FIG. 1 is a block diagram of a pumping assembly in accordance with theinvention; and

FIG. 2 is a curve representative of the operation of the pumpingassembly.

FIG. 1 is thus a block diagram of a pumping assembly comprising asecondary pump 1 having a drive motor 2 having its inlet side connectedto an enclosure in which a high vacuum is desired, and having its outletside connected to a primary pump 4 having a drive motor 5, said primarypump 4 outputting to the atmosphere.

The pumping assembly shown is, for example, portable and cordless, andtherefore includes a storage battery 6 for powering the assembly. Thebattery feeds an electrical control circuit 7 which includes, interalia, a DC-AC converter for providing a 3- phase AC to the motors 2 and5. Lines 8 and 9 represent these power supply connections.

As is known, the secondary pump 1 cannot operate unless below a certainpressure P₁ referred to as the priming pressure. Thus, when the assemblyis initially started, only the primary pump 4 is switched on, and thesecondary pump is started automatically when the pressure upstream fromthe primary pump falls below said pressure P₁. It is known that thecurrent taken by the drive motor 5 is an increasing function of inletpressure. Thus, the secondary pump is switched on when the current takenby the drive motor 5 drops below a value which corresponds to saidpriming pressure P₁. To this end, the control circuit 7 includes acurrent-sensitive relay, for example, switching at a predetermined valueof the current taken by the line 9.

According to the invention, a passive tank 10 followed by an isolatingvalve 11 are interposed between the outlet 12 from the secondary pump 1and the inlet 13 to the primary pump 4. The passive tank 10 is merely acavity having a certain volume, that is why it is called "passive".

The control circuit 7 includes a relay which operates between two valuesof the current taken by the drive motor 2 on the secondary pump 1: amaximum value I₁ and a minimum value I₂, which values correspond to twovalues of the pressure P in the isolating tank 10: the first valuecorresponding to the priming pressure P₁, and the second valuecorresponding to a pressure P₂ <P₁. The pressure P₂ corresponds to avalue P_(l) for the pressure in the vacuum enclosure 3. This pressureP_(l) is the limiting inlet pressure for the secondary pump 1.

Thus, once the pressure in the tank 10 reaches the value P₂, the controlcircuit 7 closes the valve 11 via the line 14 and switches off the drivemotor 5 of the primary pump 4. Conversely, when the pressure in theisolating tank 10 rises to the value P₁ by virtue of the secondary pump1 continuing to operate and the walls of the enclosure 3 degassing, thecontrol circuit 7 reopens the isolating valve 11 and switches back onthe primary pump 4. The pressure in the tank 10 drops again to the valueP₂, thereby switching off the primary pump 4 again and reclosing theisolating valve 11. The pressure in the isolating tank 10 thusoscillates between the two values P₁ and P₂, so that during a firstperiod of time both pumps are in operation and during a second period oftime only the secondary pump is in operation.

FIG. 2 shows this operation.

From time 0 to time t₁, the pumping assembly is started up and only theprimary pump 4 is in operation. At time t₁, the pressure in the tank 10reaches the value P₁ and the secondary pump 1 is switched on. At thismoment, the current taken by its drive motor 2 is at a maximum and isequal to I₁. The pressure falls down to P₂ at time t₂, with the currenttaken by the motor 2 also falling down to its minimum value I₂, therebytriggering the relay so that the primary pump 4 is stopped and the valve11 is closed. From time t₂ to t₃, only the secondary pump is inoperation. At t₃, the primary pump is restarted and the valve 11 isreopened, etc. . . . From t₃ to t₄, both pumps are in operation, from t₄to t₅, only the secondary pump 1 is in operation . . .

If the pumping flow Q is defined as the product of its volume rate Smultiplied by the pressure P of the pumped flow, then Q=PS.

It is specified above that the pressure P₂ is the pressure in the tank10 when the inlet side of the secondary pump 1 reaches its limitingpressure P₁. At this moment, conditions are steady, and the flow Qpumped through the primary pump 4 is equal to the outgassing flow Q₁ inthe enclosure 3.

At this moment, the flow pumped by the primary pump is Q=P₂ ·S=Q₁, whereS is the volume rate of the primary pump 4. This gives P₂ =Q₁ /S.

The ratio of on-time to off-time for the primary pump 4 is directlyrelated to the degassing flow Q₁ in the enclosure 3 and to the magnitudeof the volume V of the tank 10. These two magnitudes are related by thefollowing equation:

    P.sub.1 -P.sub.2 =ta.Q.sub.1 /V

where:

ta is the stop time of the primary pump 4 (i.e. t₃ -t₂ or t₅ -t₄ in FIG.2). Thus:

    ta=V(P.sub.1 -P.sub.2)/Q.sub.1.

Thus, the stop times increase with increasing volume V in the tank 10,with increasing priming pressure P₁ for the secondary pump 1, and withdecreasing degassing flow Q₁ from the enclosure 3.

In addition, the on-time tm of the primary pump 4 (corresponding totimes t₂ -t₁ or t₄ -t₃ or t₆ -t₅ in FIG. 2) depends on the volume V ofthe tank 10 and on the volume rate S of the primary pump 4.

These quantities are related by the following equation:

    tm=2.3(V/S)log(P.sub.1 /P.sub.2).

Thus, the on-time of the primary pump 4 decreases with decreasing volumeV of the tank 10, with decreasing pressure ratio P₁ /P₂, and withincreasing volume rate S of the primary pump 4.

This gives: ##EQU1##

Thus this ratio decreases with decreasing degassing flow Q₁ from theenclosure, with decreasing ratio P₁ /P₂, with increasing volume rate Sof the primary pump, and with increasing pressure difference P₁ -P₂.

By way of example, if the volume rate S of the primary pump 4 is S=3.6m³ /h=1 liter/second, the degassing flow Q₁ =10⁻² mb.liter/second, themaximum priming pressure P₁ =40 mb, and the minimum pressure P₂ =4.10⁻³mb, then tm=9.2 seconds and ta=4000 seconds, giving:

    tm/ta=2.3/1000 tm/(tm+ta)=2.3/1002.3≈2.3×10.sup.-3

Thus, the energy consumed by the primary pump 4 in such a pumpingassembly during a period of time t during which the assembly is in usecorresponds to 2.3×10⁻³ times the amount of energy that would have beenconsumed by the primary pump if the primary pump 4 had been operatingthroughout the period t, instead of operating intermittently. Theprimary pump operates permanently as from time t₁.

The advantage of the invention is thus clear, particularly when usedwith a cordless assembly powered by a battery.

The invention is also applicable to cases where the primary pump 4 is afixing pump, e.g. a static pump of the zeolite or "molecular sieve"type. Pumping by capturing molecules is effective only at very lowtemperature and this type of pump requires a powerful cooling system,e.g. based on liquid nitrogen circulation.

In this case, there is no drive motor 5, since the motor is replaced bythe cooling system. The control circuit 7 thus operates by switching onand off the cooling circuit 5 under the same conditions as it switcheson and off the drive motor for a rotary pump that delivers to theatmosphere.

We claim:
 1. A pumping assembly for obtaining a high vacuum, theassembly comprising a primary pump (4) and a secondary pump (1)associated in series, the inlet of the secondary pump (1) being takenfrom an enclosure (3) to be evacuated, the assembly further includingmeans (7) for starting the secondary pump (1) when the pressure upstreamfrom the primary pump (4) drops below a value P₁, the assembly beingcharacterized in that a passive tank (10) followed by an isolating valve(11) are interposed between the outlet (12) from the secondary pump (1)and the inlet (13) to the primary pump (4), and in that it includescontrol means (7) for closing the isolating valve (11) and stopping theprimary pump (4) when the pressure in said passive tank (10) reaches avalue P₂ <P₁, and for opening the isolating valve (11) and restartingthe primary pump (4) when the pressure in said passive tank (10) returnsto the pressure P₁.
 2. A pumping assembly according to claim 1,characterized in that said primary pump is a fixing pump (4) providedwith a cooling device (5), said means for stopping the primary pump (4)acting on the cooling device (5).